1. Field of the Invention
The present invention relates to a process for reducing particulate oxide-containing material, in particular fine ore, in at least one whirl bed, wherein the oxide-containing material, by means of reducing gas flowing from bottom to top, is maintained in a whirl layer, thus being reduced. The invention also relates to a plant for carrying out the process.
2. Description of the Related Art
A process of this type is known, for instance, from U.S. Pat. No. 2,909,423, WO 92/02458 and EP 0 571 358. There, reduction of the oxide-containing material, for instance, fine ore, is effected in a whirl layer maintained by reducing gas within a whirl-layer reduction reactor, wherein the reducing gas, which is injected into the whirl-layer reduction reactor via a tuyere grid, flows through the reduction reactor from bottom to top, whereas the oxide-containing material passes the reduction reactor approximately in the transverse flow relative to the reducing gas flow. A certain speed of the reducing gas within the whirl layer zone is required for maintaining the whirl layer.
On account of the relatively high speed of the reducing gas, superfines of the oxide-containing material and reduced oxide-containing material, resulting from the reduction process, are being carried away from the whirl layer, whereupon the superfines will then be contained in the reducing gas. In order to eliminate such superfines from the reducing gas--so that, on the one hand, the partially oxidized reducing gas can be further used, for instance, for preceding reduction reactors and, on the other hand, the otherwise lost oxide-containing material or already reduced material will be recovered. The reducing gas containing said superfines is conducted through dust separators, such as cyclones, whereupon the dust separated out is recycled into the whirl layer. The dust separators or cyclones preferably are arranged within the reactors (cf. U.S. Pat. No. 2,909,423); but they also may be installed outside of the reactors.
In practice, it has been shown that partially reduced or completely reduced fine-grained particles of the oxide-containing material tend to adhere or cake to one another and/or to the walls of reactors or cyclones as well as to connection ducts and conveying ducts. These phenomena are called "sticking" or "fouling", respectively. Sticking or fouling is dependent on the temperature and on the degree of reduction of the oxide-containing material. Due to the partially or completely reduced oxide-containing material adhering to, or depositing on, the walls of the reduction reactors or other plant parts, failures may occur such that it is not possible to operate the plant continuously over an extended period of time without stoppage. It has been shown that the plant must be stopped every three to four months.
Removal of the deposits and cakings is very labor-intensive and involves high costs, i.e., labor and costs arising from the loss of production of the plant. Frequently, such deposits get separated automatically, thus either falling into the whirl layer and interfering with the reduction process or--in case of deposits separating from a cyclone--causing obstruction of the dust recycling channels that lead from the cyclone to the whirl layer and hindering further dust separation from the reducing gas.
The invention aims at avoiding these disadvantages and difficulties and has as its object provide a process of the type defined above, as well as a plant for carrying out the process, to through which the reducing of particulate oxide-containing material is feasible over a very long period of time without any risk of operational interruptions caused by sticking or fouling.